1. Field of the Invention
The present invention relates to an apparatus for measuring distance with ultrasound with the aid of several ultrasonic transducers arranged preferably in a motor vehicle, in which the transit time of respective ultrasonic pulses from the ultrasonic transducers to an object and back is measured.
2. Prior Art
Ultrasonic distance measuring devices are used to an increased extent for detection of obstacles, especially in parking. This type of device comprises a control unit, which essentially contains a microcomputer with a suitable program, an ultrasonic transducer acting as a sensor and a suitable warning device. The ultrasonic transducer is, for example, arranged in the bumper.
The ultrasonic transducer propagates ultrasonic pulses when activated by the control unit. The device is then prepared to detect reflected echoes of the ultrasonic pulses. The distance to an obstacle or object is calculated from the transit time of the ultrasonic pulses from the ultrasonic transducer to the obstacle or object and back. The distance to the obstacle and/or a warning that the distance to the obstacle has dropped below a predetermined distance is reported optically and/or acoustically to the driver with the help of a connected warning device.
In the known devices sensors, namely the ultrasonic transducers, are assembled in groups which do not mutually interfere with each other because of the spatial arrangement in the bumper or in relation to the obstacle and are controlled simultaneously when ultrasonic pulses are propagated. The evaluation of the echoes in the known devices occurs continuously separately after transmission by the ultrasonic transducers. Usually the signals of the transmitting ultrasonic transducers (direct-echoes) and both ultrasonic transducers next to the transmitting ultrasonic transducers (cross-echoes) are evaluated. Thus not only the presence of an obstacle, but also its position and distance from the motor vehicle, can be determined with these known devices.
The sensitivity and thus the range of the ultrasonic distance measuring apparatus is determined essentially by the amplitude of the propagated ultrasonic pulses, the sensitivity of the receiver of the reflected ultrasonic pulses and the reflection properties of the obstacle. It is about a meter with the currently known devices. This range has been established in practice but is however also in many cases too small.